Method of controlling semiconductor device fabrication

ABSTRACT

A semiconductor wafer fabrication metrology method in which process steps are characterised by a change in wafer mass, whereby during fabrication mass is used as a measurable parameter to implement statistical process control on the one or more of process steps. In one aspect, the shape of a measured mass distribution is compared with the shape of a predetermined characteristic mass distribution to monitor the process. An determined empirical relationship between a control variable of the process and the characteristic mass change may enable differences between the measured mass distribution and characteristic mass distribution to provide information about the control variable. In another aspect, the relative position of an individual measured wafer mass change in a current distribution provides information about individual wafer problems independently from general process problems.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/811,855,filed Jul. 7, 2010, which is the National Stage of InternationalApplication No. PCT/GB09/00043, filed Jan. 7, 2009; which claimspriority benefit of UK Application No. 0800227.1 filed Jan. 7, 2008(which are hereby incorporated by reference).

FIELD OF THE INVENTION

This invention relates to semiconductor wafer metrology.

BACKGROUND TO THE INVENTION

Microelectronic devices are fabricated on semiconductor wafers using avariety of techniques, e.g. including deposition techniques (CVD, PECVD,PVD, etc.) and removal techniques (e.g. chemical etching, CMP, etc.).Semiconductor e.g. silicon wafers may be further treated in ways thatalter their mass e.g. by cleaning, ion implantation, lithography and thelike.

Depending on the device being manufactured, each wafer may be passedsequentially through hundreds of different processing steps to build upand/or to remove the layers and materials necessary for its ultimateoperation. In effect, each wafer is passed down a production line. Thenature of semiconductor manufacturing means that certain processingsteps or sequences of steps in the production flow may be repeated in asimilar or identical fashion. For example, this may be to build upsimilar layers of metal conductors to interconnect different parts ofthe active circuitry.

The cost and complexity of the processing steps required to produce acompleted silicon wafer together with the time that it takes to reachthe end of the production line where its operation can be properlyassessed has led to a desire to monitor the operation of the equipmenton the production line and the quality of the wafers being processedthroughout processing so that confidence in the performance and yield ofthe final wafers may be assured.

A single semiconductor wafer may include many different devicesmanufactured by many different techniques. For example, logic and memorydevice may be created using CMOS fabrication methods, whilst otherbipolar and compound semiconductor devices may be created using adifferent type of planar transistor fabrication technology. In future,several devices may be regularly fabricated together on a single chip ormade separately and mounted on a common platform and connected together.Future devices may also use 3D integration techniques, where the devicesare connected through the complete wafer.

It is known to use Statistical Process Control (SPC) to monitorvariability in the many processes involved in fabricating a typicalsemiconductor device. This may involve, for any given process step,setting upper and lower limits for one or more measurable parameterswhich are indicative of the result of the process step based on a normaldistribution and mean and measuring those parameters at one or onemeasurement sites on a sample (or on each) of the semiconductor waferssubject to the process step to ensure that they fall within the setlimits. Ellipsometry measurements can be used for SPC purposes.

A development of SPC that is used in some process steps is AdvancedProcess Control (APC), which is able to use measurements to adjust theprocess. SPC measurements may be used as feedback to adjust a givenprocess for subsequent wafers. Alternatively, SPC measurements may beused to feed forward information e.g. to adjust later process steps fora given wafer to compensate for variations in an earlier process step.

However, many existing metrology techniques used for SPC are limited bytheir ability to measure certain types of material or indeed certainprocesses. For example, ellipsometry can only measure opticallytransmissive films; it is not possible to use it with opaque films.Similarly, resistivity probes can only measure metals (not dielectrics).Furthermore, both of these techniques are limited to use with depositedlayers; they cannot measure etched or recessed structures.

Another typical limitation of known SPC measurement techniques is thedifficulty of measuring product wafers. In some cases, special areas(test sites) are built into the design of a wafer to check theperformance of a process step. One problem with such areas is that theyare not necessarily representative of what happens on real devicestructures and are therefore of limited value. Test wafers are analternative solution, wherein similar processes are carried out on thetest wafer structures and measured with the premise that if the processmeets specification on the test wafer, then the process will meetspecification on the product wafer. However, the increasing cost andcomplexity of wafer fabrication means that scrapped product wafers andtest wafers are becoming uneconomical. Furthermore, test sites on wafersare undesirable as they use up value space, i.e. reduce the number ofdevices that may be fabricated, hence impacting on productivity.

In WO 02/03449 the present inventors disclosed a way to measure changesin mass very accurately when semiconductor wafers are processed. Acommon feature of many semiconductor device fabrication steps is thatmaterial will be added or removed. In WO 02/03449, it was suggested thatSPC could be applied to deposition process steps using the accurate massmeasurement method.

SUMMARY OF THE INVENTION

The present invention is a development of ideas disclosed in WO02/03449.

As mentioned above, many wafer fabrication (e.g. wafer treatment) stepsinvolve a change in mass of the wafer. The inventors have discoveredthat because each individual process is unique for a given device typeand structure they can be assigned a ‘mass fingerprint’ e.g.corresponding to an average or expected or ideal mass change. Forexample, a deep silicon etch process might have a mass fingerprint of65.53 mg (based on a mean value) and a shallow silicon etch processmight only have a mass fingerprint of 3.24 mg. Even though the sameprocesses may be used for different devices types, the mass fingerprintswill change because of different, layouts in the devices and changes tothe exposed etch areas.

If a given process for a given wafer type and device type is specified,then under ideal conditions (i.e. in conditions with no systematicerror) its mass fingerprint will exhibit a distribution (referred toherein as a characteristic distribution) which is representative of theperformed process. This is true for any process carried out duringsemiconductor manufacture.

However, the inventors have also realised that the relationship of ameasured mass distribution to the characteristic distributioncorresponding to a mass fingerprint and the location of individualmeasurements relative to a characteristic distribution can be indicativeof properties of the device, i.e. properties of the element fabricatedby the process having the fingerprint. In other words, a comparison of ameasured mass change distribution for a current batch of wafers with apredetermined characteristic mass change distribution for a process mayyield useful information about the process that may otherwise bedifficult to detect. For example, mass measurements may be related tothickness, layer uniformity, stoichiometry, stress and refractive index.During fabrication, there will be variations in the process and for eachof these parameters there will be a corresponding characteristicdistribution. The inventors have understood that all of thesedistributions can be captured by measuring only the distribution of themass fingerprint for the process. Moreover, if an empirical relationshipbetween one or more parameters and the mass fingerprint is determined,it may be possible to detect behaviour of the one or more parametersfrom the measured mass fingerprint distribution. It may thus be possibleto link the behaviour of the measured mass change distribution to acertain effect whereby adjustment of the process is targeted atcorrected for that effect. Certain behaviour of the measured mass changedistribution may indicate the onset of an error. In one embodiment thisaspect of the invention may provide a means of detecting problems in thesystem which are flagged by changes in parameters that can only bemeasured directly in a destructive manner. The determined empiricalrelationship permits the evolution of a distribution of non-destructivemeasurements of mass change to be used to indicate behaviour of aparameter that can only be measured destructively. For example, it isnot possible to perform non-destructive measurements of stress on thinSiN films formed on product wafers. However, a previously establishedempirical relationship between a stress distribution and itscorresponding characteristic mass change may permit changes in stress tobe detected as changes in a measured mass change distribution.

In practice, processes may be controlled based purely on a comparison ofa measured mass change distribution with the characteristic mass changedistribution, i.e. without any direct calculation or conversion to acorresponding parameter distribution. Indeed, problems such as a reactorpressure change or a need for cleaning may manifest themselves in manyparameters. Such problems may be detected in the invention by comparingproperties of the measured mass change distribution (e.g. skew, drift ofmean, broadening or narrowing) itself with the characteristic masschange distribution to assess the process.

A particular advantage of the proposed method is that it may permitadjustments to the process to be determined and implemented before apredetermined process control limit is breached. In other words, themethod may be used to identify mass measurement patterns which fallwithin the control limits of a given distribution but which indicate theonset of problems in the process.

Alternatively or additionally, the location of an individual mass changemeasurement (e.g. for a single semiconductor wafer) within a currentmass change distribution may provide information about that wafer.Herein a current mass change distribution may be a mass changedistribution consisting of measured mass changes of a plurality ofwafers subjected to the process before the individual mass changemeasurement is taken. Whereas in known SPC techniques measurements ofone or more semiconductors wafers are compared with static controllimits, e.g. set in fixed positions relative to a characteristicdistribution, in the invention the relative position of a measurementwithin a distribution is detected. This relative position may beindicative of problems with that individual wafer. Such problems may beindependent of problems that affect the behaviour of the distribution asa whole. The invention may thus facilitate detection of problems thatwould otherwise go undetected. For example, in a situation where themean of a distribution slips below the characteristic mean, a highoutlier in the current distribution may fall within the control limitsof the characteristic distribution and hence be deemed acceptablewhereas a comparison with the current distribution may indicate that aproblem with that wafer has occurred.

Remedial action may be taken to address problems detected in individualwafers, e.g. by adjusting parameters of that process to ensure thatsubsequent wafers do not repeat the problem (feedback) or of a futureprocess to compensate in that wafer (feed forward). In other words, theinvention may be used as part of an APC system. In one aspect, thecomparison may be used to enable future process steps to compensate forvariations in a given process step. In another aspect, the comparisonmay facilitate identification of failed or defective wafers if it isdesirable to tighten a measured distribution for a given process step.

At its most general, the invention provides a metrology method forsemiconductor wafer fabrication in which one or more process steps arecharacterised by a change in wafer mass, whereby during fabrication massis used as a measurable parameter to implement statistical processcontrol on the one or more of process steps. One advantage of thismethod is that the same measuring equipment may be used to monitor aplurality of different process steps. Changes in one or more differentcontrol variables that affect the process may manifest themselves in ameasured mass change distribution. Important variables may include layerthickness, layer uniformity, stoichiometry, stress and refractive index.The invention may permit changes in these variables to be detected,either implicitly through monitoring behaviour of a measured mass changedistribution, or explicitly by establishing an empirical relationshipbetween a control variable and the characteristic mass change, wherebybehaviour of the measured mass change distribution is indicative ofbehaviour of the control variable.

According to one aspect of the invention, there may be provided a methodof statistical process control (SPC) including: obtaining acharacteristic mass change distribution for a semiconductor waferfabrication process; measuring a change in mass of a plurality ofsemiconductor wafers subjected to the process; comparing the shape of ameasured mass change distribution consisting of the measured masschanges of the plurality of semiconductor wafers with the shape of thecharacteristic mass change distribution to monitor the process. Thisaspect may provide a way of monitoring the effectiveness of anyfabrication process step by comparing a ‘current’, i.e. recentlyobtained, distribution with a characteristic, e.g. ideal, distributionto identify problems in the process. For example, the mean of thecurrent distribution may stray from the mean of the characteristicdistribution. This can be an indicator that the process is notperforming efficiently and appropriate remedial action can beundertaken. An advantage of the method is that the remedial action canbe taken in an efficient manner, i.e. before an unsatisfactory wafer isproduced by the process but not so often as to unnecessarily disrupt thefabrication process.

In this aspect, properties of the distributions are compared rather thanmaking comparisons using individual measurements. For example, comparingthe shape of the measured mass change distribution with the shape of thecharacteristic mass change distribution may include detecting any one ormore of relative broadening, relative narrowing or skew between themeasured mass change distribution and the characteristic mass changedistribution.

Alternatively or additionally, the method may include obtaining a meanfor the measured mass change distribution and comparing the obtainedmean with the mean of the characteristic distribution.

The measured mass change distribution at a point in time consists of aplurality of mass changes measured in a period immediately before thatpoint in time. The measured mass change distribution may consist of apredetermined, e.g. fixed, number of measured mass changes, or all ofthe measured mass changes taken in a predetermined period before thepoint in time. The method may include periodically updating the measuredmass change distribution, e.g. by selecting a more recently measuredplurality of mass changes or setting a new point in time. Updating themeasured mass change distribution enables its behaviour, e.g. evolutionin time, to be monitored.

In another aspect, the invention may provide a method of statisticalprocess control (SPC) including: measuring a change in mass for each ofa plurality of semiconductor wafers subjected to a semiconductor waferfabrication process; at a point in time, obtaining a current measuredmass change distribution consisting of a plurality of the measured masschanges which were measured in a period immediately before the point intime; subsequently measuring a change in mass of one or moresemiconductor wafers subjected to the process after the point in time;and comparing the subsequently measured change in mass with the currentmeasured mass change distribution to monitor the process. In thisaspect, the relative position of an individual measurement in a currentdistribution is determined. This relative position may be indicative ofa problem with that particular wafer and hence may be dealt with in adifferent manner from problems detected by the comparison of themeasured mass change distribution with the characteristic mass changedistribution.

The measured mass change distribution may be obtained in the same manneras for the first aspect.

Comparing the subsequently measured change in mass with the currentmeasured mass change distribution may include determining a differencebetween the subsequently measured change in mass and the mean of thecurrent measured mass change distribution. This may permit outliers ofthe current distribution to be detected.

In a development of the idea, the method may include determining anempirical relationship between a control variable and the characteristicmass change of the process; and obtaining an indication of the behaviourof the control variable based on the empirical relationship and thecomparison of the shape of the measured mass change distribution withthe shape of the characteristic mass change distribution. The controlvariable may be a property of the measured wafer, e.g. relating tomaterial added (e.g. deposited) or removed by the process, e.g. layerthickness, layer uniformity, doping concentration, moisture content,stoichiometry, stress and refractive index. Knowledge of how thebehaviour of the measured mass change distribution is affected by thechanges in the control variable may be used to adjust the process forsubsequent wafers (e.g. if the control variable for a wafer indicatesthat the fabrication apparatus needs cleaning, etc.) or to alter futureprocess steps for the wafer e.g. to compensate for variation in thecontrol variable.

In one embodiment, the method may include using the empiricalrelationship to set an upper control limit and a lower control limit formass change measurements.

The characteristic mass change distribution may result from a detailedinvestigation of a process before that process is incorporated intofabrication of product wafers. The investigation may involve obtaining acorrelation between variations in the process and mass change. Duringthe investigation, the variations in the process may be definitivelymeasured e.g. using destructive techniques not suitable for productwafers. In this way the characteristic mass change distribution can beused effectively as a means to match non-destructive mass changemeasurements (i.e. measurements suitable for use on product wafers) withproperties of the process that cannot be determined directly in anon-destructive manner. However, obtaining such links is not essential:changes in the measured mass change distribution may be linked directlythrough experience with problems with the process.

For example, when depositing materials (e.g. hafnium-silicon oxides)with a high dielectric constant (k) value for a CMOS gate application,the stoichiometry of the deposited layer is a key component affectingproperties and behaviour of the layer. It is desirable to monitor theprocess in order to ensure that the deposited layer has the desiredcharacteristics. However, since high k films are opaque they cannot bemeasured with ellipsometry. Moreover, where thin (e.g. approximately 3nm [30 Å]) gate thicknesses are used, such films do not lend themselvesto analysis from techniques such as X-ray Reflectivity (XRR) orRutherford Backscattering Spectroscopy (RBS), which typically need afilm thickness of at least 20 nm (200 Å). Also, these latter techniquesare destructive and cannot be used on product wafers.

The present invention may be used to overcome this problem. In oneembodiment an empirical relationship may be obtained between thestoichiometry and the characteristic mass change by comparing thecomposition of the high k film (i.e. at % of the three constituents Hf,Si and O) with the film density. Additionally, the allowable tolerancefor thickness (and uniformity, etc.) for the various film compositionsmay be investigated. All of these factors affect the mass changedistribution for the process, so based on the investigations it ispossible to assign a band of mass changes which have a high probability(e.g. >95%, preferably >99%) of being caused by a process which depositsa layer with a satisfactory composition and thickness. Theinvestigations described above may demonstrate what faults in theprocess can cause the mass change to lie above or below the allowedband. This can help identify a probable cause of a mass change outsidean allowed band which either enables immediate identification of anon-ideal product wafer or permits correction of the problem duringsubsequent treatment steps. This concept may be applied to the positionof measured mass changes within the allowed band, to fine-tunesubsequent treatment steps. Thus, establishing the characteristic masschange distribution may involve investigating a process step todetermine one or more relationships between process parameters and masschange to enable upper and lower control limits to be set.

Another example of a process to which the present invention may beapplied is a gate etch. A typical tolerance value for a gate etch isless than 4 nm across a 300 mm diameter wafer. Directly observing a 45nm gate width (e.g. using a scanning electron microscope) to decidewhether the process is within a set specification is difficult, timeconsuming and destructive. However, since there can be 1000 km of gatelength on a 300 mm wafer, a nanometre-scale change in width of thestructure can provide a measurable change in mass. In this case,establishing an empirical relationship between gate width and thecharacteristic mass change may involve producing a number of gate etcheswhich are observed through a scanning electron microscope (SEM) and havetheir mass changes measured to determine a relationship between masschange and gate width. The relationship may be expressed graphically. Ina development of the idea, the relationship may even be extrapolated togain insight on mass changes for critical dimensions whose variationsare too small to be directly observed through a SEM.

Measuring the change in mass of the semiconductor wafers may includedetermining a difference between mass values obtained before and afterthe process. The mass values may be obtained by measuring the weight ofeach wafer in a weighing chamber and compensating for buoyancy exertedon each wafer by the atmosphere in the chamber. Compensating foratmospheric buoyancy may be achieved in any of the manners disclosed inWO 02/03449. A typical 300 mm wafer weighing about 128 g can experiencea buoyancy force equivalent to about 45 mg. The magnitude of this forcecan vary over a relative short time by 10-20% (i.e. 4-6 mg).

The invention may be applicable to a single removal (e.g. etch) ordeposition process, or a combination of process steps, since a pluralityof individual steps will still exhibit a characteristic mass changedistribution. For example, the invention may be applicable to variousprocesses in the Back End of the Line (BEoL) stage of fabricating a DualDamascene structure. The invention may also be used to monitor physicalvapour deposition (PVD) of thin films (i.e. films having a thickness ofless than 50 nm [500 Å]), monitor the creation of stacked films (ofdielectric and/or metallic material) for gate or capacitor structures,monitor the fabrication of blind or deep trenches (e.g. in fabricatingDRAM structures), or to monitor shallow etch processes or polymerremoval steps. The invention may be used to monitor doping concentration(e.g. where mass change is caused by adding Ar, P or B atoms to asilicon substrate). Similarly, the invention may be able to monitor masschange caused by moisture absorption, adsorption and/or desorption fromfilms fabricated on a substrate. The moisture content of film with a lowk-value can be an important factor. In one embodiment, the invention maybe used to determine the extent of thermal treatment required for adeposited dielectric material (e.g. spin-on wet materials) to bring itsmoisture content into a desired range. The invention may also be used tomonitor a cleaning process used in device manufacture preparation, wherea very thin layer (e.g. less than 1 nm [less than 10 Å]) is removed froma silicon substrate e.g. by known wet chemistry techniques. The masschange in this case may be very small, so to improve accuracy, aplurality of measurements may be taken for each wafer, and an average ofthose measurements used for comparison with the characteristic masschange distribution. This technique may be generally applicable forprocesses where the mass change is very small, i.e. at or beyond therepeatability limit of a measuring instrument.

In a further development, the method of the invention may includecomparing the indication of the control variable obtained via massmetrology with information about the process, preferably informationabout the control variable, obtained by another (non-mass) metrologymeasurement. The other metrology measurement may also be non-destructiveso that it can be used effectively on product wafers. Using the massmetrology technique in combination with another metrology output mayenable anomalies or errors to be picked up that would otherwise remainundetected. For example, ellipsometry can be used to determine thethickness and uniformity of certain types of film. In one embodiment,ellipsometry data may show that a deposited film measured at a specificlocation (i.e. a flat area) is good, but the mass change measurement maynot lie in the expected band, which may indicate that there is a problemat a location not measurable by ellipsometry, i.e. there may be aproblem with film coverage in a gap where device fabrication is takingplace. Such problems may have gone undetected if ellipsometry was usedalone.

Aspects of the invention discussed above may be combined to provide amethod in which distributions are compared to detect long term processproblems and in which relative positions of individual measurementswithin a distribution are determined to detect individual problemwafers.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention are described in detail below with referenceto the accompanying drawings, in which:

FIG. 1 is a schematic flow diagram showing mass changes associated withvarious processes to which the present invention may be applied;

FIG. 2 is a detailed flow diagram showing mass changes associated withindividual process steps in a typical front end of line (FEoL)semiconductor wafer fabrication process to which the present inventionmay be applied;

FIG. 3 is a schematic representation of a characteristic mass changedistribution for use in the present invention;

FIGS. 4A and 4B are graphs showing monitoring of a measured mass changedistribution relative to a characteristic distribution that is anembodiment of the invention;

FIG. 5 is another graph monitoring of a measured mass changedistribution relative to a characteristic distribution that is anotherembodiment of the invention;

FIGS. 6A and 6B are schematic diagrams illustrating the effect ofmechanical stress in MOS materials;

FIGS. 7A and 7B are schematic diagrams illustrating how a nitrideoverlayer can impart stress to cause tension or compression in a MOSchannel;

FIG. 8 is a cross-sectional view of a MOSFET structure with a highstress nitride film applied thereto;

FIG. 9 is a schematic cross-sectional view of stress-imparting filmsapplied to an n-MOS and a p-MOS device respectively;

FIG. 10 is a graph showing a relationship between stress imparted andSi—N ratio of various silicon nitride films deposited at differenttemperatures;

FIG. 11 is a graph showing a relationship between stress imparted anddensity of various silicon nitride films;

FIG. 12 is a graph showing a relationship between dark current anddensity of various deposited silicon nitride films;

FIG. 13 is a graph showing monitoring of a measured mass changedistribution for a silicon nitride film depositing step relative to acharacteristic distribution that is an embodiment of the invention;

FIG. 14 is a schematic cross-sectional view through a semiconductordevice the fabrication of which may be monitoring using the presentinvention;

FIGS. 15A to 15C are schematic flow charts showing fabrication of aninter-metal dielectric (IMD) layer;

FIG. 16 is a schematic flow chart showing how the present invention canbe applied to the fabrication process shown in FIG. 15;

FIG. 17 is a graph showing mass changes for the process steps shown inFIG. 15 for different exposed areas;

FIG. 18 is a schematic cross-sectional view through a dual damascenesemiconductor device the fabrication of which may be monitoring usingthe present invention;

FIGS. 19A-D are schematic flow charts showing fabrication of a trenchlayer in the dual damascene semiconductor device shown in FIG. 18;

FIG. 20 is a schematic flow chart showing how the present invention canbe applied to the fabrication process shown in FIG. 19; and

FIG. 21 is a graph showing mass changes for the process steps shown inFIG. 19 for different exposed areas.

DETAILED DESCRIPTION

Further Options and Preferences

FIG. 1 illustrates schematically the idea that every step in asemiconductor fabrication process involves a change in mass. Moreover,each process will have a characteristic mass change with its owndistribution. The chart in FIG. 1 shows how the mass (on the y-axis, notto scale) of a semiconductor wafer may change according to the type offabrication process it experiences. Thus, a plasma enhanced chemicalvapour deposition (PECVD) increases mass, chemical mechanical polishing(CMP) and etch processing decrease mass, and physical vapour deposition(PVD) and atomic layer deposition (ALD) increase mass. Each of the masschanges has a different magnitude. An accurate mass measurement may beable to detect these changes to generate a measured mass changedistribution for a plurality of semiconductor wafers which are treated.By comparing individual mass change measurements or the measured masschange distribution to a characteristic mass change distribution for agiven process step it is possible to monitor that process.

FIG. 2 is a similar chart to FIG. 1 but this time shows a typicalprocess flow for a real situation, in this case a FEoL process flow fora 130 nm logic device. The inventors have realised that the samemetrology technique (indeed the same metrology apparatus, if convenient)can be used to permit statistical process control (SPC) or advancedprocess control (APC) to be performed on each step in this process flow.This is a step beyond conventional SPC or APC, which is limited by itsmeasurement technique (e.g. ellipsometry) to certain types of process ofmaterial.

Fundamentally, the process control proposed herein revolves around acomparison between a characteristic mass change distribution obtained inadvance for a given process step and measured mass values taken duringactual fabrication. The mass change caused by a given process will varynaturally from wafer to wafer due to random variables that cannot becontrolled. The characteristic mass change distribution isrepresentative of that normal distribution and may be obtained byexperiment. FIG. 3 is a graph of data obtained from such an experiment.Measured mass change is plotted on the x-axis and frequency on they-axis. The several thousand measurements are taken to yield a normaldistribution. A mean value (i.e. a mass change fingerprint value) andthe standard deviation σ may also be calculated.

FIG. 4A is a graph showing how the characteristic mass changedistribution can be used in one embodiment of the invention. In thisgraph real mass change measurements taken during a deep silicon etchprocess are plotted; the measured mass change is on the y-axis and thewafer number (in order of treatment) is on the x-axis (and with a z-axisin FIG. 4B). The plotted points therefore trace the behaviour of theactual process over time. In this embodiment, the characteristic masschange distribution is used to set an upper specification limit 10, alower specification limit 12, an upper control limit 14 and a lowercontrol limit 16. Each of these are mass values corresponding to certainpoints on the characteristic mass change distribution. For example, ifthe mean value of the mass change distribution is x, the upper and lowerspecification limits 10, 12 may be x+3σ and x-3σ respectively. The upperand lower control limits 14, 16 may be provided within the specificationlimits to allow a margin of measurement error. All four limits arerepresented as horizontal lines on the graph in FIG. 4A. In thisembodiment, a measurement taken outside of the specification zonedefined by the upper and lower specification limits 10, 12 (e.g.measurement 18) indicates a failure or a fault in the process step. Thewafer corresponding to that measurement may be rejected. Measurementstaken inside the specification zone but outside the control zone definedby the upper and lower control limits 14, 16 may be tagged for furtherinvestigation, e.g. to try to improve the measured distribution for theprocess.

In this embodiment it is also possible to monitor behaviour of themeasured mass change distribution over time, in particular the movementof the mean measured mass change value (indicated by dashed line 20 inFIG. 4A). In this embodiment the mean measured mass drifts (the masschange is gradually decreasing over time). This may be due to polymerbuild up in the chamber where the process step is taking place. As shownin FIG. 4A, the chamber is cleaned following a measurement 18 indicatinga failure. In a modification of this embodiment, the chamber clean maybe triggered by the measured mass change mean reaching a predeterminedthreshold value. An advantage of this could be reduction in failurerate. Thus, it can be seen in general how process problem can beenidentified (and perhaps solved) using measurements of actual productwafers without scrapping any wafers and without using any test wafers.

FIG. 5 is a graph showing how a characteristic mass change distributioncan be used in another embodiment of the invention. In this embodiment,the process is atomic layer deposition of a thin TaN layer. The graphplots measured mass change (y-axis) against wafer number x-axis) in thesame way as FIG. 4A. The characteristic mass change distribution is usedto provide upper and lower specification limits 30, 32 and upper andlower control limits 34, 36 as in FIG. 4A.

In this embodiment, a group of measurements 38 lower than the lowerspecification limit are measured. This may mean that the deposited layeris thinner or that the etch which produced the profile for the TaN tocover has less surface area (i.e. a previous etching step removed lessmaterial). Either way, there is a problem which has been identified onproduct wafers which otherwise would have gone undetected. Conventionalmetrology techniques provide no way to measure a 5 nm (50 Å) TaN layeron a product wafers.

In the embodiments described herein, the mass measurements may becompensated to take account of atmospheric buoyancy. Thus, themeasurements may take place in an apparatus comprising a wafer holderand weighing instrument enclosed in a chamber. The wafer holder may belocated in an upper portion of the chamber and the weighing instrumentin a lower portion. The two portions may be partitioned (with a throughhole for a connecting member) in a known manner (e.g. WO 02/03449) toreduce the volume of the wafer holder enclosure and thereby reduce aircurrents. The chamber may contains a temperature sensor, a humiditysensor and a pressure sensor. The sensors are mounted such that theirsensing elements are located in the upper portion of the chamber, withthe wafer holder. The pressure sensor may be a Druck PMP4010AB. Thetemperature and humidity sensors can be combined, e.g. as a Pico RH02.The measurements taken by these sensors are fed to a processing unit,e.g. an external PC or internal microprocessor, to allow the air densityto be calculated, e.g. using

$\rho_{AIR} = \frac{{0.3485P} - {0.00132 \times \left( {{0.0398T^{2}} - {0.1036T} + 9.5366} \right) \times H}}{\left( {273.14 + T} \right) \times 1000}$where ρ_(air) is the density of air in g/cm3, P is the pressure in mBar,T is the temperature in ° C. and H is the relative humidity expressed asa percentage. The air density can be used to calculate the effect ofatmospheric buoyancy on the wafer using the equation:

$B = \frac{W_{w} \times \left( {\frac{\rho_{air}}{\rho_{w}} - \frac{\rho_{air}}{\rho_{c}}} \right)}{\left( {1 - \frac{\rho_{air}}{\rho_{w}}} \right)}$where B is the atmospheric buoyancy effect in grams, W_(w) is the weightof the wafer sensed by the weighing instrument (in grams), ρ_(air) isthe calculated air density in g/cm3, ρ_(w) is the wafer density ing/cm3, and ρ_(c) is the density (in g/cm3) of a calibration weight usedto calibrate the weighing instrument.

FIGS. 6 to 13 illustrate the use of the present invention to monitor theeffects of silicon nitride (SiN) layers that are fabricated in themanufacture of a semiconductor device. SiN films are used forpassivation of semiconductor device before they are packaged and toprovide strain forces in strained gate applications. In bothapplications the repeatability of process performance is critical foroptimum device performance. The present invention provides a monitoringtechnique that can be used with actual product wafers (i.e. it does notrequire test wafers or a test site on a product wafer) and which canidentify failing devices immediately from the measurement, i.e. withoutrequiring additional testing steps.

FIGS. 6A and 6B respectively illustrate the mechanical stresses in aconventional nMOS and pMOS devices (shown schematically) which canenhance carrier mobility in their channels. The strongly anisotropicsensitivity of mobility to strain is known. The magnitude of stress in aSiN contact etch stop layer (CESL) has a direct effect of the drivecurrent obtainable in the channel of a nMOS or a pMOS device. Thediagrams in FIGS. 7A and 7B show how a bi-axially strained nitrideoverlayer can transfer tension from the overlayer into the source/drainand hence into the channel. In the upper diagram in FIG. 7A theoverlayer is tensile to cause uniaxial tension in the channel. In thelower diagram in FIG. 7B the overlayer is compressed to cause uniaxialcompression in the channel. The magnitude and direction of the straincan be controlled through process strain engineering.

FIG. 8 is a cross-sectional view of a MOS structure in which a highstress nitride overlayer 40 is fabricated over the gate electrode 46. Acover layer 44 of NiSi and a spacer 42 separates the top and sides ofgate electrode 46 respectively from the overlayer 40; the stress istransferred from the overlayer 40 to the cover layer 44 and spacer 42.

FIG. 9 illustrates schematically a cross-sectional view through asemiconductor device in which an nMOS 50 having a tensile contact etchstop layer (tCESL) 54 is located next to a pMOS 52 having a compressivecontact etch stop layer (cCESL) 56.

The stress imparted by an SiN overlayer can be controlled to becompressive or tensile by controlling parameters of the depositionprocess. FIG. 10 shows the relationship between Si—N ratio and stress ina deposited SiN layer for different deposition temperatures. It can beseen that silicon-rich SiN provides can provide tensile films. However,while it is relatively easy directly to measure stress in thick filmsand blanket wafers, such techniques are not transferable to productwafers, which typically have much thinner SiN layers.

The present inventors realized that it is possible to use a relationshipbetween stress and film density to obtain an indirect indication ofstress by using the mass metrology technique of the present invention.FIG. 11 is a graph useful for determining an empirical relationshipbetween a distribution of stress in a SiN film and the mass of that film(i.e. the mass change involved in depositing that film). The graph inFIG. 11 shows that compressive SiN films have a higher density thantensile films, which indicates that a variation in stress can be mappedonto a variation in mass. This empirical relationship may be used toprovide an indication of how a change is stress is manifested in adifference in shape between a measured mass change distribution and thecharacteristic mass change distribution for the deposition of a SiNlayer. The measured mass change distribution may be considered to be, inpart, representative of the stress in the deposited film. Control limitsas discussed above may be set so that measured mass changes on productwafers can be used for SPC of the SiN layer deposition.

As mentioned above, silicon nitride films are often used for passivationof a semiconductor device before it is packaged. The purpose ofpassivation is to minimize dark current from flowing in the staticdevice. SiN-coated wafers have less dark current than non-passivatedwafers. However, in passivated wafers dark current flow can befacilitated by the presence of hydrogen in the SiN film. Control of thedeposition process can reduce the hydrogen content, but there has notbeen an easy way to monitor hydrogen content or dark current on actualproduct wafers.

FIG. 12 is a graph which shows a relationship between dark current anddensity of an SiN passivation layer. Based on this relationship, theinventors have realized that it is possible to obtain an indirectindication of hydrogen content (i.e. improved dark current inhibition)by using the mass metrology technique of the present invention. Controllimits as discussed above may be set so that measured mass changes onproduct wafers can be used for SPC of the SiN layer deposition. Thecharacteristic mass change distributions (and any associated controllimits) for the passivation and strained layer applications may bedifferent from each other, even though the actual process of depositingSiN is similar.

FIG. 13 is a graph showing how a characteristic mass change distributioncan be used in the SiN deposition embodiments discussed above. The graphplots measured mass change (y-axis) against wafer number (x-axis) in thesame way as FIGS. 4A and 5. The characteristic mass change distributionis used to provide upper and lower control limits 60, 62. This methodcan be used to resolve changes of 0.2 nm (2 Å) in the thickness of a SiNlayer or 0.05 gcm-3 in density of such a layer.

In addition to individual measurements being assessed for the likelihoodof failure (i.e. if they are outside the control limits something isprobably wrong), the measured mass change distribution itself may bemonitored with a view to detecting long term changes in the process(similar to the polymer build up example given above). The system may beadapted automatically to adjust for or react to detected changes, e.g.by altering deposition conditions or requiring a reset of the apparatus.

FIGS. 14 to 17 illustrate the use of the present invention to monitorthe thickness of an oxide inter-metal dielectric (IMD) layer fabricatedin Back End of Line (BEoL) processing.

FIG. 14 is a schematic cross-sectional view through a semiconductordevice which has a plurality of function levels arranged on top of oneanother. Each function level includes a layer of metal, e.g. patternedaluminium lines. An IMD layer is provided at the interface betweenadjacent levels to isolate the metal layers from one another. It isdesirable for the IMD layers to be thin to save space but tick enough toprevent conduction. In practice, a deposited oxide layer is subjected tochemical mechanical polishing (CMP) to reduce its thickness and generatea flat surface for use as a base for the next metal layer.

FIGS. 15A-C shows the process steps involved in fabricated an IMD layer.

FIG. 15A shows a semiconductor structure 100 that is the product of FEoLprocessing. Metal (e.g. aluminium) lines 102 connected to the drain andsource electrode and a metal (e.g. aluminium) line 104 connected to agate electrode are fabricated as a first metal layer. A mass measurementM.sub.0 is obtained for this intermediate structure, e.g. using theatmospheric buoyancy compensation technique mentioned above or by anyother known way.

FIG. 15B shows the semiconductor structure 100 after the high densityplasma (HDP) deposition of an oxide layer 106 has taken place. Thedeposited oxide layer 106 has a thickness T_(HDP) of around 1600 nm,which completely covers the metal layer lines 102, 104. A massmeasurement M₁ is obtained for this intermediate structure, the mass ofthe deposited oxide layer 106 therefore being calculable asM_(DEPOSIT)=M₁−M₀.

FIG. 15C shows the semiconductor structure 100 after a CMP process hasremoved a top portion 108 of the oxide layer 106. The treated oxidelayer 106 has a thickness T_(OXIDE) of around 1300 nm, which completelycovers the metal layer lines 102, 104. The top surface of the treatedoxide layer is flat, thereby providing a planar region for supportingsubsequent layers. A mass measurement M₂ is obtained for this structure,the mass of the top portion 108 removed by polishing therefore beingcalculable as M_(CMP)=M₁−M₂, and the remaining oxide layer asM_(OXIDE)=M₂−M₀.

The final thickness of the oxide layer thus depends on two independent(and technically dissimilar) process steps. In the absence of an etchstop, it can be difficult to determine whether the correct level ofoxide has been removed. Variation in the final thickness of the IMDlayer can cause device integration problems and in some cases can affectthe device's performance.

The mass values obtained in this process may be used to perform SPC andAPC (or feed forward process control) to help identify and/or correctfor potential problems. Since M_(DEPOSIT) is directly proportional tothe thickness of the deposited oxide layer 106, it is possible to usethe position of this value relative to a characteristic mass changedistribution for the deposition step to adjust the duration of the CMPstep. For example, if M_(DEPOSIT) is above a+1σ point on thecharacteristic mass change distribution, the CMP tool may instructed toincrease the removal time (according at a given removal rate) for thatwafer. Similarly, if M_(DEPOSIT) is below a-σ point on thecharacteristic mass change distribution, the CMP tool may instructed todecrease the removal time. The values of M_(OXIDE) may also be comparedwith a characteristic mass change distribution for the total IMDfabrication process (i.e. the combination of HDP deposition and CMP) tomonitor the thickness of the resulting IMD layers.

FIG. 16 is a flow chart of an APC process similar to that outlinedabove. Step S1 represents obtaining the mass measurement M.sub.0 for thesemiconductor structure 100 after FEoL processing. Step S2 is the HDPdeposition of oxide layer 106 and subsequent measurement of M₁ andcalculation of M_(DEPOSIT). Step S3 is forward the value of M_(DEPOSIT)to the CMP apparatus, where it may be compared with the characteristicmass change distribution for M_(DEPOSIT) to judge whether or not tocorrect the CMP removal time. Step S4 is the correction of the CMPremoval time (if necessary) followed by carrying out the actual CMP.Step S5 is measuring M₂ after completion of CMP and calculatingM_(OXIDE) and comparing that value with a characteristic mass changedistribution for the combined HDP deposition and CMP process to monitorthe thickness of the IMD layer.

FIG. 17 is a graph showing how deposited masses and IMD layer masses mayneed to vary depending on how much metal is present in the metal layerto be covered. Where there is little metal (so a large exposed area)more mass needs to be deposited to make up the difference in volume.Accordingly, each IMD layer may have its own characteristic massdistribution which in part may depend on the configuration of the metallayer which it covers. The graph in FIG. 17 shows that it may bedesirable for the CMP polishing to remove a consistent amount ofmaterial. Monitoring the value of M_(CMP), i.e. the relationship betweenM₁ and M₂ is one way to achieve this.

FIGS. 18 to 21 illustrate the use of the present invention to monitor atrench etch in a dual-Damascene process. Conventionally the etch processwas controlled by an etch stop layer, but it is desirable to omit thislayer to enable faster devices to be produced.

FIG. 18 is a schematic cross-sectional view through a semiconductordevice which has a plurality of function levels arranged on top of oneanother. In this case, the base 150 of the structure is the result FEoLprocessing and may including one or more nMOS and/or pMOS devices. Afirst single-Damascene layer 152 is formed directly on top of the base150. Four dual-Damascene layers 154, 156, 158, 160 are fabricated on topof the first single Damascene layer. A second single-Damascene layer 162is provided on top of the dual-Damascene layers, and the structure iscompleted by a subtractive aluminium layer 164 on top of the secondsingle-Damascene layer 162.

In each dual-Damascene layer two different structures (e.g. a trenchbetween a pair of vias in dual Damascene layer 154) are etched into aninsulating layer (e.g. silicon oxide). The etched structures areoverfilled with metal (e.g. copper) which is then subjected to CMP toexpose the insulating layer and hence reveal the conductive patterningformed by the etched structures.

Etch stop layers where conventionally used to ensure that a trench etchwas performed to the correct depth, i.e. that the relative depths of atrench and via were fabricated properly.

FIG. 19 shows the process steps involved in applying the method of thepresent invention to the etching processes which fabricate the twodifferent etching structures in an insulating layer as part of adual-Damascene process. In this example, the layer being fabricated isthe first dual-Damascene layer 154 shown in FIG. 18. The method ishowever applicable to each dual-Damascene layer. As each dual-Damascenelayer may have a different configuration, the characteristic mass changedistribution for the respective etch processes may be different.Comparing a measured mass change for each step with the characteristicmass change distribution may give an indication of whether the etch hasbeen performed properly. Furthermore, APC may be performed by measuringa mass change corresponding to the deposition of the insulating layer,comparing this with a characteristic mass change distribution for thatdeposition process and adjusting the subsequent etch processaccordingly. For example, a thinner insulating layer can be subjected toless etching whereas a thicker insulating layer can be subjected to moreetching.

FIG. 19( a) shows the semiconductor base 150 that is the product of FEoLprocessing. The first single-Damascene layer is fabricated on a topsurface of the base 150. Metal (e.g. copper) lines 151 connected to thedrain and source electrode and a metal (e.g. copper) line 153 connectedto a gate electrode are fabricated as a first metal layer. A massmeasurement M₀ is obtained for this intermediate structure, e.g. usingthe atmospheric buoyancy compensation technique mentioned above or byany other known way.

FIG. 19( b) shows the structure after the deposition of a low-kdielectric (insulating) oxide layer 166. A mass measurement M.sub.1 isobtained for this intermediate structure, the mass of the depositedoxide layer 166 therefore being calculable as M_(LOWK)=M₁−M₀. The valueof M_(LOWK) may be compared with a characteristic mass changedistribution to control subsequent etching steps, as discussed withreference to FIG. 20 below.

FIG. 19( c) shows the structure after three vias 168 are etched throughto some of the metal lines 151, 152 in the single-Damascene layer 152. Amass measurement M₂ is obtained for this structure, and the mass removedby the etch may therefore being calculable as M_(VIA)=M_(LOWK)−M₂. Thevalue of M_(VIA) may be compared with a characteristic mass changedistribution to ensure that the etch process has been performedcorrectly.

FIG. 19( d) shows the structure after a trench 170 is etched between twoof the vias 168. A mass measurement M₃ is obtained for this structure,and the mass removed by the etch may therefore being calculable asM_(TRENCH)=M_(VIA)−M₃. The value of M_(TRENCH) may be compared with acharacteristic mass change distribution to ensure that the etch processhas been performed correctly.

FIG. 20 is a flow chart showing an embodiment of the invention appliedto the process steps discussed with reference to FIG. 19. Step S10 ismeasuring a value of M₀ for the intermediate structure comprising thesemiconductor base 150 and the first single-Damascene layer 154. StepS11 is depositing the low-k dielectric layer 166, measuring M₁ andobtaining (e.g. calculating) M_(LOWK). Step S12 is performing SPC usingthe value of M_(LOWK), e.g. by plotting M_(LOWK LOWK) on a chart andcomparing with a characteristic mass change distribution e.g. bycomparing with control limits set according to that characteristicdistribution. The relative position of M_(LOWK) in the characteristicmass change distribution may be indicative of process performance, e.g.can be used to as an indicator of problems in layer thickness, k-value,uniformity, etc. Step S13 is performing first APC by feeding forward thevalue of M_(LOWK) to adjust etch process parameters, e.g. etch timing,temperature or the like, for the via etch process. In other words, therelative position of within its corresponding characteristic mass changeM_(LOWK) distribution can be indicative of the thickness of theinsulating layer, which is directly relating to a required etch depth.By feeding forward the information in this way, the etching process canbe made more effective, i.e. fewer wafers may be lost due to failurescaused by variations in fabrication. Step S14 is etching the vias,measuring M.sub.2, obtaining (e.g. calculating) M_(VIA) and thenperforming SPC using the value of M_(VIA), e.g. by plotting M_(VIA) on achart and comparing with a characteristic mass change distribution e.g.by comparing with control limits set according to that characteristicdistribution. Step S15 is performing second APC by feeding forward thevalue of M_(LOWK) (possibly also with the value of M_(VIA)) to adjustetch process parameters, e.g. etch timing, temperature or the like, forthe trench etch process. In the same way that M_(LOWK) is used as anindicator of thickness to control the via etch, it can be used in thisstep to control the depth of the trench relative to the vias. Step S16is etching the trench, measuring M₃, obtaining (e.g. calculating)M_(TRENCH) and then performing SPC using the value of M_(TRENCH), e.g.by plotting M_(TRENCH) on a chart and comparing with a characteristicmass change distribution e.g. by comparing with control limits setaccording to that characteristic distribution.

FIG. 21 is a graph showing how mass changes for the various processesdiscussed above may need to vary depending on the size of the etchedstructures. Where the vias and trench are small less mass needs to beremoved from a given insulating layer to make up the difference involume. Accordingly, each dual-Damascene layer may have its owncharacteristic mass distribution which may depend on the configurationof its etched structures.

I claim:
 1. A method of statistical process control (SPC) forsemiconductor wafer fabrication comprising a plurality of fabricationprocesses, the method including the steps of: obtaining a characteristicmass change distribution for each one of the plurality of fabricationprocesses; measuring a change in mass for each fabrication process forone or more monitored semiconductor wafers subjected to the plurality offabrication processes; comparing each measured change in mass to thecharacteristic mass change distribution of its respective fabricationprocess; and generating for each monitored semiconductor wafer processcontrol data representative of the position of each measured change inmass relative to its respective characteristic mass change distributionfor each one of the plurality of fabrication processes completed forthat monitored semiconductor wafer.
 2. A method according to claim 1,wherein obtaining the characteristic mass change distribution at a pointin time for each one of the plurality of fabrication processes includesthe steps of: measuring a change in mass for each of a plurality ofsemiconductor wafers subjected to the fabrication process in a periodimmediately before the point in time; generating a current measured masschange distribution consisting of the changes in mass which weremeasured in the period immediately before the point in time; and usingthe current'measured mass distribution as the characteristic mass changedistribution for monitored semiconductor wafers subjected to thefabrication process after the point in time.
 3. A method according toclaim 2, wherein the current measured mass change distribution consistsof a predetermined number of measured mass changes.
 4. A methodaccording to claim 2, wherein the current measured mass changedistribution consists of all mass changes measured in a predeterminedperiod immediately before the point in time.
 5. A method according toclaim 2, including periodically updating the measured mass changedistribution.
 6. A method according to claim 1 including determiningwhether or not a monitored semiconductor wafer is to progress to afuture fabrication step based on the generated process control data.